North Korean leader Kim Jong Un called for bolstering nuclear forces this year during a visit to the country's nuclear material production base and nuclear weapons institute, state media KCNA said on Wednesday.

South Korea 's Yonhap News Agency said North Korea might be using a vertical cold-launch system to fire the missiles. The system, which is commonly associated with submarine- and ship-based weapons, ejects a missile from its container before the missile ignites midair.

North Korea has test-fired sea-to-surface strategic cruise guided missiles, the North's state media reported Sunday, quoting Kim as saying that Pyongyang's deterrence means are being perfected.

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North Korea's foreign ministry vowed the "toughest counteraction" against the United States as long as Washington "refuses" the country's sovereignty.

North Korea announced it had performed a test of its cruise missile system on Saturday. It was the communist country’s fourth known missile launch since November and the second in less than a week since President Donald Trump’s inauguration.

Kim Jong Un Vs Trump | Kim Jong Un Nuclear | US News | North Korea News As Donald Trump eyes a potential return to the White House, North Korea's Kim Jong Un is ramping up tensions by showcasing his nuclear arsenal through recent missile tests.